General intelligence generally refers to intellectual ability, which is conceptually somewhat different from the general factor g or fluid reasoning ability. In the psychometric tradition, it is widely accepted that there are two related but distinct components of general intelligence, referred to as “fluid (gF) and crystallized (gC) general intelligence” (Cattell 1963; 1987, Theory of fluid and crystallized intelligence: A critical experiment. J. Educ. Psychol., 54, 1-22; Cattell, R. B. (1987). Intelligence: Its structure, growth and action. New York: Elsevier Science). gF generally refers to reasoning and novel problem-solving ability, to be able to see relationships, as in analogies and letter and number series, which is independent from prior experience and learned knowledge. In contrast, gC is cognitive functioning based on previously acquired knowledge available in long-term store, including semantic knowledge and episodic memory.
Since the early 20th century, many psychometric batteries (i.e., IQ tests; for example the Wechsler-derived batteries, the Thorndike test, the Kaufman test, the Raven test) have been devised to measure individual differences in general intelligence. Using the batteries, psychometricians have attempted to clarify sources of individual differences, ultimately in order to enhance the cognitive functions. However, the mechanisms still are unclear because psychometric researches provide the correlational evidence rather than the causal evidence (Neisser, U., Boodoo, G., Bouchard, T. J. J., Boykin, A. W., Brody, N., Ceci, S. J., et al. (1996). Intelligence: Knowns and unknowns. Am. Psychol., 51, 77-101).
The RAPM (Raven's Advanced Progressive Matrices Set II), a standard test for general intelligence, is one of the purest measures of psychometric g (Raven, J., Raven, J. C., & Court, J. H. (1998). Manual for Raven's Progressive Matrices and Vocabulary Scales. Oxford: Oxford Psychologists Press). The WAIS (Wechsler's Adult Intelligence Scale), a standard intelligence quotient (IQ) test, is a measure of both crystallized intelligence (gC) and fluid intelligence (gF) based on eleven subtests (Marshalek, B., Lohman, D. F., & Snow, R. (1983). The complexity continuum in the radex and hierarchical models of intelligence. Intelligence, 7, 107-127; Wechsler, D. (1981). WAIS-R Manual. New York: Psychol. Corp): Information, Comprehension, Vocabulary, Similarities, Block Design, Object Assembly, Picture Completion, Digit Span, Arithmetic, Digit Symbol, and Picture Arrangement. Factor analytical studies of WAIS found the presence of factors named Verbal Comprehension and Perceptual Organization, and demonstrated that Information, Comprehension, Vocabulary, and Similarities subtests are classified into strong measures of Verbal Comprehension or gC, and Block Design, Object Assembly, and Picture Completion subtests are categorized into strong measures of Perceptual Organization or gF (Beck, N. C., Horwitz, E., Seidenberg, M., Parker, J., & Frank, R. (1985). WAIS-R factor structure in psychiatric and general medical patients. J Consult Clin Psychol, 53(3), 402-405; Leckliter, I. N., Matarazzo, J. D., & Silverstein, A. B. (1986). A literature review of factor analytic studies of the WAIS-R. J Clin Psychol, 42(4), 332-342; Marshalek, B., Lohman, D. F., & Snow, R. (1983). The complexity continuum in the radex and hierarchical models of intelligence. Intelligence, 7, 107-127; McGrew, K. S., & Flanagan, D. P. (1998). The Intelligence Test Desk Reference (ITDR): Gf-Gc Cross-Battery Assessment. Boston: Allyn & Bacon; Parker, K. (1983). Factor analysis of the WAIS-R at nine age levels between 16 and 74 years. Journal of Consulting and Clinical Psychology, 51, 302-308).
Over the last decade, neuroimaging studies using various techniques including anatomical MRI (Magnetic Resonance Image), fMRI (functional MRI), PET (Positron Emission Tomography), and MRS (Magnetic Resonance Spectroscopy) rapidly have unveiled the neurobiological bases of diverse cognitive functions such as fluid reasoning, working memory, and problem-solving ability (Gray, J. R., & Thompson, P. M. (2004). Neurobiology of intelligence: science and ethics. Nat. Rev. Neurosci., 5, 471-482). However, this approach appears to have some intrinsic limitations to differentiate the neural basis of gC from gF or the unitary factor g. First, individual differences in gF and gC exhibit robust intercorrelation in the normal cohort (r=0.7-0.8, Jensen, A. R. (1998). The g factor: The science of mental ability. Westport, Conn.: Praeger; Kaufman, A. S., & Horn, J. L. (1996). Age changes on test of fluid and crystallized ability for women and men on the Kaufman adolescent and adult intelligence test (KAIT) at ages 17-94 years. Archives of Clinical Neuropsychology, 11, 97-121). Their relation could be explained by the notion that gF plays a substantial role in encoding and retrieving information in long-term store and thereby in facilitating the accumulation and expression of gC, although there are distinct neural bases for these two functional domains (gF and gC) of intelligence. Second, the typical crystallized knowledge content of WAIS (Wechsler's Adult Intelligence Scale) subtests Information and Vocabulary reveals high g-loadings (r=0.6-0.7) despite low reliance on fluid reasoning ability and working memory capacity (Colom, R., Jung, R. E., & Haier, R. J. (2006). Distributed brain sites for the g-factor of intelligence. Neurolmage, 31(3), 1359-1365; Lee, K. H., Choi, Y. Y., Gray, J. R., Cho, S. H., Chae, J. H., Lee, S., et al. (2006). Neural correlates of superior intelligence: stronger recruitment of posterior parietal cortex. Neurolmage, 29(2), 578-586).
Therefore, to dissect the neural mechanism specific for crystallized knowledge, more sophisticated experimental paradigms and methods are required.
The present inventors have formulated a combined model of gF and gC that accounts for dissociation of gC and gF, and further developed a better method for predicting individual differences in general intelligence, thereby completing the present invention.